Process for sintering silicon nitride compacts

ABSTRACT

Sintered silicon nitride articles are prepared by sintering a silicon nitride compact of a density of at least 1.3 g/cm3 in nitrogen gas atmosphere at substantially atmospheric pressure, while the compact is embedded in a protective powder comprising silicon nitride, boron nitride, or a mixture of silicon nitride and boron nitride, and one or more sinterization additives for silicon nitride, said additives being present in the protective powder in an amount of from 3 to 20% by weight.

This is a continuation of application Ser. No. 970,592, filed Dec. 18,1978, now abandoned.

The present invention relates to a process for sintering silicon nitridecompacts.

Silicon nitride is currently considered, among ceramic materials, to bethat with the most interesting prospects for use in the manufacture ofstructural components for heat engines, i.e., gas turbines. Hot-pressingof silicon nitride yields material of high mechanical properties, butarticles of complex shapes are not readily obtained by this method whichhas, moreover, limited production capability. Therefore, various studieson the sintering of silicon nitride have been initiated to produce highstrength, high density silicon nitride in complex shapes on a costeffective basis.

It is known that silicon nitride is a compound which is very difficultto sinter both because of the covalent nature of its bonding and becauseof its thermal instability at temperatures higher than 1500° C. In orderto activate the densification process it is necessary to treat thecompound at temperatures higher than 1500° C., with consequent shiftingof the equilibrium

    Si.sub.3 N.sub.4 ⃡3Si+2N.sub.2                 ( 1)

towards the formation of the elements.

At 1700° C. in the presence of nitrogen, at atmospheric pressure, theloss in weight of the Si₃ N₄ powder may be of the order of 20% per hour.

Several studies have been carried out on the possibility of limitingthis decomposition; positive results have, until now, been obtainedsolely by operating in a nitrogen atmosphere and at a pressure (100 atm)such as to allow the equilibrium of the reaction (1) to be displacedtowards the left.

Densification can also be promoted by increasing the "green" density ofthe compacts to be sintered, by using a powder of finer particle size,or by using sintering additives. Since silicon nitride is not readilysinterable in the pure state, additives have been used in the art topromote sintering, usually by liquid phase formation. Reference is madein this connection to G. E. Gazza, Sintered Silicon Nitride, Ceramicsfor High Performance Applications--II, Proceeding of the Fifth ArmyMaterials Technology Conference, Newport Mar. 21-25, 1977, Editors J. J.Burke, E. N. Lenoe and R. N. Katz, published in cooperation with theMetals and Ceramics Information Center, Columbus, Ohio. Conventionalsintering additives are usually chosen from oxides such as MgO, Y₂ O₃,CeO₂, BeO and Z₂ O₂, rare earth based additions, spinel and non-oxideadditions, such as Mg₃ N₂, AlN and Mg₂ Si, as shown in the abovepublication.

As well as inhibiting the decomposition of the Si₃ N₄ during thesintering treatment, it is thus necessary to control the possibleremoval or redistribution within the Si₃ N₄ of the sintering additives.These latter generally form liquid phases at the treatment temperaturewhich, on the one hand, may be partially removed due to their vapourpressure, and, on the other hand, may react with the free siliconproduced by the decomposition of the Si₃ N₄, being redistributed in anon-homogeneous manner in the material.

The object of the present invention is to provide a sintering processwhich limits the thermal decomposition of the Si₃ N₄ at the sinteringtemperature without the need to use pressure greater than atmosphericand which provides conditions which allows sintered products to beobtained with high density and structural homogeneity.

The invention provides a process for preparing a sintered siliconnitride article, which comprises the step of sintering a silicon nitridecompact of a density of at least 1.3 g/cm³ in nitrogen gas atmosphere atsubstantially atmospheric pressure, while the compact is embedded in aprotective powder comprising Si₃ N₄, BN or a mixture of silicon nitrideand boron nitride, and one or more sinterization additives for siliconnitride, said additives being present in the protective powder in anamount of from 3 to 20% by weight.

Thus, a main feature of the process of the invention consists ofembedding the Si₃ N₄ compact to be sintered in a protective powder of aspecified type and of a suitable composition. Without wishing to bebound to any theory on the reaction mechanisms, it is probable that thepowder partially decomposes and vaporizes during the densificationtreatment, creating an atmosphere of N₂ around the compact and formingvapour or liquid phases which diffuse into the compact or preventsimilar phases present in the compact from diffusing towards the surfaceby filling up the pores or compensating for concentration gradientsresulting from evaporation or from chemical reactions.

It should be noted that the use of protective powders consisting ofsilicon nitride and/or boron nitride makes possible acceptable controlof the thermal dissociation reaction of the Si₃ N₄ during treatment inthe temperature range of 1600°-2000° C.

The materials thus obtained, however, have little uniformity ofappearance and have structural and/or chemical discontinuities betweenthe core and the external surface.

The innovative factor lies in the use, as the protective powder, of Si₃N₄ or a mixture of Si₃ N₄ and BN containing one or more of theconventional sintering additives used for activating the densificationprocess. By this means, the thermal decomposition of the Si₃ N₄ islimited and in many cases rendered practically negligible and sinteredmaterials which are structurally and chemically homogeneous areobtained.

Any conventional sintering additive may be used for the purpose.Preferred additives are MgO, Y₂ O₃, CeO, ZrO₂, BeO, Mg₃ N₂ and AlN.Other examples of additives are Mg₂ Si, spinels, such as MgAl₂ O₄, andrare earth additions, such as La₂ O₃. Although not being classifiedamong the conventional sintering additives, Fe can also be used withadvantage as an additive in the process of the invention, especially inmixture with conventional additives such as MgO, Y₂ O₃ and CeO₂.

The protective powder may also contain release agents chosen fromrefractory materials. Since BN acts as a release agent, the powderpreferably consists essentially of Si₃ N₄, BN and additives, although apowder consisting essentially of Si₃ N₄ and additives may also be usedwith advantage.

The amount of additives in the protective powder is normally from 3 to20% by weight, preferred values being generally of from 5 to 15% byweight and best results being generally obtained with values of theorder of 5-10%, the most suitable values depending also on the nature ofthe silicon nitride to be sintered. Moreover, the best results aregenerally obtained by using MgO as an additive, either alone or inmixture with other additives such as Y₂ O₃.

The silicon nitride compact may be embedded in the powder by placing thecompact on a layer of powder, which has been introduced into thecontainer (generally a graphite, Si₃ N₄ or SiC crucible), andsubsequently covering it completely with the said powder. Preferably,the powder should have a homogeneous density and a uniform compositionaround the compact.

The container, which is closed by a lid, is then generally subjected toflushing with nitrogen to remove the gases trapped in the protectivepowder and in the interface between the latter and the compact, beforethe sintering treatment. The degassing treatment may be carried out atatmospheric pressure. It is also possible to carry out the degassing byapplying a subatmospheric pressure (for example 10⁻³ Torr) and thenflushing with nitrogen while gradually bringing the pressure toatmospheric. The degassing may conveniently be carried out whilegradually bringing the compact to the sintering temperature. If desired,a series of vacuum and flushing treatments may be carried out.

The sintering treatment is carried out in a nitrogen atmosphere, thepressure being substantially atmospheric. The other sintering conditionsdo not substantially depart from those conventionally used in the art.The sintering temperature generally does not exceed 2000° C. andpreferably is from 1600° to 1900° C., best results being generallyobtained with temperatures of the order of 1800° C. The sintering timeis generally of the order of 0.5-6 hours.

The material to be sintered may be produced by forming a silicon nitridepowder into compacts of the desired shape by any conventional method,such as pressing and isopressing, vibration compaction (ramming),extrusion and injection molding, cold isostatic pressing being generallypreferred. In each case, the operating conditions should be such as toobtain a compact with a density of at least 1.3 g/cm³, and preferably ofthe order of 1.9-2 g/cm³, the most suitable values depending also on thesize of the particles. Thus, in the case of particles having a size ofat least 1μ, densities lower than 1.5 g/cm³ do not yield completelysatisfactory results. On the other hand, "green" densities exceedingabout 2.1 g/cm³ are difficult to achieve. The silicon nitride particlesgenerally have a size of from 0.1 to 44μ, best results being generallyobtained with sizes not exceeding about 5μ.

The silicon nitride powder to be compacted may contain one or moresintering additives chosen from Fe and those conventionally used in theart, such as MgO, Y₂ O₃, CeO₂, ZrO₂, BeO, Mg₃ N₂, Mg₂ Si, AlN, MgAl₂ O₄,Be₃ N₂ and La₂ O₃. As in the case of the "external" additives present inthe protective powder, it is possible to use mixtures of Fe withconventional additives. Best results are generally obtained by using amixture of MgO and Y₂ O₃.

These "internal" additives are generally used in amounts not exceeding20 wt.% with respect to the compact, and preferably from 1 to 12 wt.%,best results being generally obtained with amounts of from 5 to 10% byweight, the most suitable amounts also depending on the amount ofexternal additive present in the protective powder. Thus, when theexternal additive is used in low amounts, it is generally preferable touse somewhat higher amounts of internal additives. Likewise, when theinternal additive is used in low amounts, or is absent from the compact,it is generally preferable to use somewhat higher amounts of externaladditives. The internal additives may be identical with, or differentfrom the external additives. However, it is generally preferable to useidentical additives or a mixture comprising at least one of theadditives used in the protective powder.

The material to be sintered may also consist of reaction bonded siliconnitride. As is known in the art, reaction bonded silicon nitride isgenerally prepared by forming silicon powder into a compact of thedesired shape and density (generally at least 1.3 g/cm³ and preferablyat least 1.5 g/cm³). The compact is then nitrided, generally at1350°-1450° C., with consequent increase in density without substantialdimensional change, thus obtaining a reaction bonded material havinggenerally a density of at least 2.2 g/cm³ and typically of the order of2.5-2.6 g/cm³. Densities exceeding 2.7 g/cm³ are difficult to achieve,whatever may be the starting density of the silicon compact and thenitridation conditions. This reaction bonded material can with advantagebe submitted to the sintering process of the invention, thus obtaining amaterial of improved strength and density, the final density being closeto the theoretical value (3.18 g/cm³).

The compacting of the silicon powder and the nitriding may be carriedout by any of the known methods. Reference is made in this connection toR. W. Ohnsorg, Reaction Bonded Si₃ N₄, Preparation and Properties, theAmerican Ceramic Society Spring Meeting, Washington, D.C., May 9, 1972,and F. L. Riley, Nitridation and Reaction Bonding, Nitrogen Ceramics,Noordhoff International Publishing, Leyden, The Netherlands, 1977. Inparticular, the silicon powder generally has a particle size of from 0.1to 44μ, best results being generally obtained with low sizes.

The silicon powder may contain small mounts of conventional nitridingcatalysts. According to one embodiment of the present invention, one ormore sintering additives as hereinbefore defined (additives present inthe protective powder and possibly in the silicon nitride to becompacted) are also added to the silicon powder to be converted intoreaction bonded material. The amount of additives is generally such asto ensure in the reaction bonded material an additive content notexceeding 20 wt.%, and preferably from 1 to 12 wt.%, best results beinggenerally obtained with amounts of from 5 to 10 wt.%. As in the case ofthe compacts obtained from silicon nitride powder, the most suitablevalues depend on the amount of external additive present in theprotective powder. As to the choice of these internal additives, theconsiderations are wholly similar to those already expressed in the caseof compact obtained from silicon nitride powder, and excellent resultshave also been obtained when using a mixture of MgO and Y₂ O₃ asinternal additives.

In conclusion, the process of the invention affords the followingadvantages:

possibility of sintering Si₃ N₄ compacts without substantial thermaldecomposition during the treatment;

use of sintering furnaces operating at atmospheric pressure without theneed to use autoclaves;

obtaining of homogeneous sintered articles both from the structural andthe chemical point of view;

possibility of application of the method to components of any shape andsize.

The process of the invention will now be illustrated by the followingnon-limitative Examples.

EXAMPLE 1

A silicon nitride powder having an average size of 5μ and containing 5%by weight of MgO and 2% by weight of Fe was formed into a compact havinga density of 2g/cm³ by cold isostatic pressing.

The compact was embedded in a protective powder with the followingcomposition by weight: 50% Si₃ N₄, 43% BN, 5% MgO and 2% Fe. The powderwas prepared by mixing under wet conditions and subsequent drying. Thepacking was carried out by introducing a first layer of the powder intoa graphite crucible, placing the compact on this layer and subsequentlycovering it completely with the powder, taking care that the powder beof homogeneous density and uniform distribution around the compact. Thecontainer, closed by a graphite lid, is subjected to degassing by vacuumtreatment (10⁻³ Torr) and flushing with nitrogen, to remove the gasestrapped in the protective powder and at the interface between the latterand the compact. The pressure is brought to 750 Torr and the temperatureis gradually brought to the sintering temperature, while passing purenitrogen.

The sintering treatment is carried out at 1800° C. for two hours, thepressure of the nitrogen being adjusted to maintain it constantly at theinitial value.

The compact thus sintered has a practically negligible (0.5%) loss inweight and does not have structural dishomogeneities as results frommicro-probes and micrographic controls. The sintered compact has adensity of 3.05 g/cm³.

EXAMPLE 2

The run of Example 1 was repeated, using a compact (cylinder 10 cm inheight and 5 cm in diameter) with the following composition by weight:91% Si₃ N₄, 8% Y₂ O₃ and 1% MgO, and a density of 2.0 g/cm³.

Use was made of a protective powder with the following composition byweight: 50% Si₃ N₄, 45% BN and 5% MgO.

The sintering treatment was carried out at 1800° C. for five hours.

The sintered compact had substantially the same composition and thefollowing properties.

    ______________________________________                                        Density                   3.20 g/cm.sup.3                                     Total porosity            +2%                                                 X-ray analysis            β Si.sub.3 N.sub.4                                                       amorphous phase                                                               traces of SiC                                       Bending strength at 25° C.                                                                       60 Kg/mm.sup.2                                       950° C.           60 Kg/mm.sup.2                                      1100° C.           37 Kg/mm.sup.2                                      1250° C.           39 Kg/mm.sup.2                                      1400° C.           11 Kg/mm.sup.2                                      Weibull modulus (25° C.)                                                                         9.3                                                 Young modulus, E, at 25° C.                                                                      240,000 MN/m.sup.2                                  1000° C.           240,000 MN/m.sup.2                                  1100° C.           233,000 MN/m.sup.2                                  1200° C.           231,000 MN/m.sup.2                                  1300° C.           224,000 MN/m.sup.2                                  Thermal conductivity at 25° C.                                                                   41.9 W/mk                                           400° C.            27.5 W/mk                                           Oxidation in statical air after                                               100 hours at 1000° C.                                                                            0.1 mg/cm.sup.2                                     1100° C.           0.28 mg/cm.sup.2                                    1200° C.           0.70 mg/cm.sup.2                                    1300° C.           2.50 mg/cm.sup.2                                    1350° C.           8.20 mg/cm.sup.2                                    ______________________________________                                    

EXAMPLE 3

The run of Example 2 was repeated, using the same protective powder anda reaction bonded silicon nitride compact of the same size with the samecomposition by weight (91% Si₃ N₄, 8% Y₂ O₃ and 1% MgO). The compact wasprepared by isostatic pressing of the silicon powder up to a density of1.6 g/m³ and nitridation. The compact had a density of 2.55 g/cm³.

The sintering treatment was carried out at 1800° C. for four hours.

The sintered compact had substantially the same composition and thefollowing properties.

    ______________________________________                                        density                   3.20 g/cm.sup.3                                     total porosity            2%                                                  X-ray analysis            β Si.sub.3 N.sub.4                                                       amorphous, traces                                                             of SiC and FeSi.sub.2                               Bending strength at 25° C.                                                                       100 Kg/mm.sup.2                                     Oxidation in statical air after                                               100 hours at 1300° C.                                                                            0.90 mg/cm.sup.2                                    1350° C.           5.20 mg/cm.sup.2                                    ______________________________________                                    

EXAMPLE 4

A mixture consisting of 90 wt.% silicon, with a maximum grain size of44μ and average grain size of 5μ, 5 wt.% MgO and 5 wt.% Y₂ O₃ is formedinto cylinders (diameter=30 mm, h=45 mm, density=67% of the theoreticalvalue) by means of cold isostatic pressing in rubber containers at apressure of 2500 Kg/cm².

These samples are nitrided in a graphite resistance furnace in anatmosphere of flowing nitrogen (5 l/min.) for a period of 100 hours, thetemperature being gradually brought from 1100° to 1390° C. with periodsof stay at intermediate temperatures.

The material thus obtained has a density of 2.55-2.6 g/cm³ (80% of thetheoretical value) with Si₃ N₄ predominantly in the α form (>80%).

The samples are submitted to a sintering treatment at 1800° C. for timesvariable from 1 to 3 hours in a graphite container, using a protectivepowder with the following composition by weight: 60% Si₃ N₄, 30% BN, 5%MgO and 5% Fe. Once the sample is embedded in the powder, vacuum isapplied (10⁻³ Torr) while the temperature is brought to 1100° C., andnitrogen is then passed and the pressure brought to 500 Torr. Thetemperature is then gradually brought to 1800° C. while bringing thepressure to 750 Torr. The sintered sample is cooled in a nitrogenatmosphere.

The samples thus sintered have the following properties.

    ______________________________________                                        density                   2.95-3.1 g/cm.sup.3                                 X-ray analysis            β Si.sub.3 N.sub.4                                                       traces of silicates                                                           and oxo-nitrogen                                                              compounds                                           porosity                  <8%                                                 bending strength (25° C.)                                                                        55-65 Kg/mm.sup.2                                   ______________________________________                                    

EXAMPLE 5

Samples of reaction bonded silicon nitride having the same density,structure and size as in Example 4 are prepared by the same procedure asin Example 4 from a silicon powder containing 5 wt.% of MgO and 2 wt.%of Fe.

Using the same procedure as in Example 4, the samples are sintered at1700°-1800° C. for times of from 1 to 3 hours, using a protective powderwith the following composition by weight: 50% Si₃ N₄, 45% BN and 5% MgO.The sintered samples thus obtained have the following properties

    ______________________________________                                        density              2.85-2.95 g/cm.sup.3                                     X-ray analysis       βSi.sub.3 N.sub.4                                                        tracess of silicates                                                          and oxo-nitrogen                                                              compounds                                                porosity             <10%                                                     bending strength (25° C.)                                                                   45-55 Kg/mm.sup.2                                        ______________________________________                                    

EXAMPLE 6

Samples of reaction bonded silicon nitride having the same density,structure and size as in Example 4 are prepared by the same procedure asin Example 4 from a silicon powder containing 5% by weight of Y₂ O₃ and2% by weight of Fe.

Using the same procedure as in Example 4, the samples are sintered at1800° C. for 2 hours, using a protective powder with the followingcomposition by weight: 50% Si₃ N₄, 40% BN, 5% MgO and 5% Y₂ O₃. Thesintered samples have the following properties.

    ______________________________________                                        density              3.05 g/cm.sup.3                                          X-ray analysis       βSi.sub.3 N.sub.4                                                        traces of silicates                                                           and oxo-nitrogen                                                              compounds                                                porosity             <5%                                                      bending strength (25° C.)                                                                   60 Kg/mm.sup.2                                           ______________________________________                                    

EXAMPLE 7

Samples of reaction bonded silicon nitride having the same density,structure and size as in Example 4 are prepared by the same procedure asin Example 4 from a silicon powder containing 8 wt.% of CeO₂ and 2 wt.%of Fe.

Using the same procedure as in Example 4, the samples are sintered at1800° C. for 2 hours, using a protective powder with the followingcomposition by weight: 50% Si₃ N₄, 40% BN; 5% MgO and 5% CeO₂.

The sintered samples have the following properties.

    ______________________________________                                        density              2.9 g/cm.sup.3                                           X-ray analysis       βSi.sub.3 N.sub.4                                                        traces of silicates                                                           and oxo-nitrogen                                                              compounds.                                               porosity             <5%                                                      bending strength (25° C.)                                                                   50 Kg/mm.sup.2.                                          ______________________________________                                    

EXAMPLE 8

Operating as in Example 4, a parallelepiped (5×5×20 mm) of reactionbonded silicon nitride predominantly in the α form and having a densityof 2.37 g/cm³ is sintered at 1800° C. for 1 hour, using a protectivepowder with the following composition by weight: 50% Si₃ N₄, 45% BN and5% MgO. The MgO has a surface area of about 35 m² /g.

The sintered sample has the following properties

    ______________________________________                                        density             3.01 g/cm.sup.3                                           linear shrinkage    6.9%                                                      variation in weight +0.1%                                                     X-ray analysis      βSi.sub.3 N.sub.4                                                        traces of FeSi.sub.2, Si                                                      and SiC                                                   Mg content          0.75%                                                     ______________________________________                                    

EXAMPLE 9

Operating as in Example 4, a parallelepiped (44×18×8 mm) of commercialreaction bonded silicon nitride having a density of 2.47 g/cm³ issintered at 1800° C. for 3 hours, using a protective powder with thefollowing composition by weight: 50% Si₃ N₄, 45% BN and 5% MgO with asurface area of about 35 m² /g.

The sintered sample has the following properties

    ______________________________________                                        density              3.05 g/cm.sup.3                                          linear shrinkage     6%                                                       variation in weight  +0.9%                                                    X-ray analysis       βSi.sub.3 N.sub.4                                                        traces of FeSi,                                                               Si and SiC                                               Mg content           0.9%                                                     ______________________________________                                    

EXAMPLE 10

A sample of commercial silicon nitride powder is milled to an averagesize of about 1 micron, and formed into cylinders (diameter=28 mm,height=45 mm) having a green density of 1.9 g/cm³ by isostatic pressing.

Operating as in Example 4, the samples are sintered at 1800° C. for 3hours, using a protective powder with the following composition byweight: 50% Si₃ N₄, 45% BN and 5% of MgO with a surface area of 38 m²/g. The rise in temperature (300° C./h) was sufficient to permitdiffusion of the additive in the samples.

The sintered material has the following properties.

    ______________________________________                                        density            3.02 g/cm.sup.3                                            linear shirikage   14.8%                                                      variation in weight                                                                              +0.8%                                                      X-ray analysis     βSi.sub.3 N.sub.4                                                        traces of FeSi, SiC                                                           and Si                                                     Mg content         1.02%                                                      ______________________________________                                    

EXAMPLE 11

A sample of commercial silicon nitride is milled to an average size ofabout 1 micron and formed into cylinders (diameter 28 mm; height 45 mm)having a green density of 1.9 g/cm³, by isostatic pressing.

Operating as in Example 4, the samples are sintered at 1800° C. for 3hours, using a protective powder with the following composition byweight: 50% Si₃ N₄, 45% BN and 5% Mg₃ N₂ with a surface area of 1.4 m²/g.

The sintered material has the following properties

    ______________________________________                                        density             2.82 g/cm.sup.3                                           linear shrinkage    10.5%                                                     loss in weight      0.4%                                                      X-ray analysis      βSi.sub.3 N.sub.4                                                        traces of FeSi.sub.2,                                                         SiC and Si                                                Mg content          0.62%                                                     ______________________________________                                    

EXAMPLE 12

A parallelepiped (5×5×20 mm) of reaction bonded Si₃ N₄ having a densityof 2.54 g/cm³ is sintered at 1800° C. for 1 hour, operating as inExample 4 and using a protective powder with the following compositionby weight: 50% Si₃ N₄, 45% BN and 5% Mg₃ N₂ with a surface area of 1.4m² /g.

The sintered material has the following properties.

    ______________________________________                                        density               2.93 g/cm.sup.3                                         linear shrinkage      2.8%                                                    variation in weight   +0.23%                                                  Mg content            0.90%                                                   ______________________________________                                    

EXAMPLE 13

The run of Example 12 is repeated by using a sintering period of 5hours. The sintered material has the following properties.

    ______________________________________                                        density               3.01 g/cm.sup.3                                         linear shrinkage      6.5%                                                    variation in weight   none                                                    Mg content            1%                                                      ______________________________________                                    

We claim:
 1. A process for increasing the density of a reaction bondedsilicon nitride article, which comprises the step of sintering thereaction bonded silicon nitride article of a density of at least 2.2g/cm³ in an atmosphere consisting essentially of nitrogen atsubstantially atmospheric pressure at a temperature of 1600° to 2000°C., while the article is embedded in a protective powder comprisingsilicon nitride, boron nitride, or a mixture of silicon nitride andboron nitride, and one or more sintering additives for silicon nitride,said sintering additives being present in the protective powder in anamount of from 3 to 20% by weight, wherein said sintering additives areselected from the group consisting of MgO, Y₂ O₃, Mg₃ N₂, CeO₂ andmixtures thereof with Fe.
 2. The process of claim 1, wherein thereaction bonded silicon nitride article to be sintered also comprisesone or more sintering additives uniformly dispersed therein in an amountnot exceeding 20% by weight, said reaction bonded silicon nitridearticle to be sintered being formed by forming silicon powder into acompact containing said sintering additives and then nitriding the sameto yield said reaction bonded silicon nitride article.
 3. The process ofclaim 1, wherein the protective powder comprises MgO.
 4. The process ofclaim 1, wherein the protective powder comprises from 5 to 15% by weightof sintering additives.
 5. The process of claim 1, wherein said compactcontains from 1 to 12% by weight of sintering additives.
 6. The processof claim 5, wherein the compact contains MgO and Y₂ O₃ sinteringadditives.
 7. The process of claim 1, wherein the sintering is carriedout at temperature from 1600° to 1900° C.
 8. The process of claim 1,wherein the protective powder is degassed before the sinteringtreatment.
 9. The process of claim 2, wherein said silicon powder has adensity of at least 1.3 g/cm³.
 10. The process of claim 2, wherein saidsilicon powder compact is nitrided at 1350° to 1450° C.
 11. The processof claim 10, wherein said nitriding of said silicon powder compactincreases the density thereof without substantial dimensional change,thereby providing said reaction bonded silicon nitride compact of adensity of at least 2.2 g/cm³.
 12. The process of claim 11, wherein thefinal density of the sintered silicon nitride article is close to thetheoretical density of 3.18 g/cm³.
 13. The process of claim 12, whereinsaid silicon powder has a particle size of from 0.1 to 44μ.
 14. Theprocess of claim 13, wherein said reaction bonded silicon nitridearticle has a density on the order of 1.5-2.6 g/cm³.
 15. The process ofclaim 1, wherein said protective powder consists essentially of saidsilicon nitride, boron nitride, or a mixture of silicon nitride andboron nitride, and said one or more sintering additives.
 16. The processfor increasing the density of a reaction bonded silicon nitride article,which comprises the step of sintering the reaction bonded siliconnitride article of a density of at least 2.2 g/cm³ in an atmosphereconsisting essentially of nitrogen at substantially atmospheric pressureat a temperature of 1600° to 2000° C., while the compact is embedded ina protective powder consisting silicon nitride, boron nitride or amixture of silicon nitride and boron nitride, and one or more sinteringadditives for silicon nitride, said sintering additives being present inthe protective powder in an amount of from 3 to 20% by weight, whereinsaid reaction bonded silicon nitride compact to be sintered alsocomprises one or more sintering additives selected from the groupconsisting of MgO, Y₂ O₃, Mg₃ N₂, CeO₂ and mixtures thereof with Feuniformly dispersed therein in an amount not exceeding 20% by weight,said reaction bonded silicon nitride compact to be sintered being formedby forming a silicon powder into a compact containing said sinteringaids and then nitriding the same to yield said reaction bonded siliconnitride article.
 17. The process of claim 1, wherein the reaction bondedsilicon nitride article product is structurally and chemicallyhomogeneous.
 18. The process of claim 1, wherein the article is embeddedin the protective powder by the steps which consist of placing thearticle on a layer of the protective powder and then embedding thearticle in the protective powder.
 19. The process of claim 1, whereinthe protective powder consists of silicon nitride and the one or moresintering additives.
 20. The process of claim 1, wherein the protectivepowder consists of boron nitride and one or more sintering additives.21. The process of claim 1, wherein the protective powder consists ofsilicon nitride, boron nitride and the one or more sintering additives.22. A process for increasing the density of a reaction bonded siliconnitride article which comprises the steps of:forming silicon powder intoa compact and nitriding the same to yield the reaction bonded siliconnitride article; sintering the reaction bonded silicon nitride articleof a density of at least 2.2 g/cm³ in an atmosphere consistingessentially of nitrogen at substantially atmospheric pressure at atemperature of 1600° to 2000° C., while the article is embedded in aprotective powder comprising silicon nitride, boron nitride, or amixture of silicon nitride and boron nitride, and one or more sinteringadditives for silicon nitride, said sintering additives being present inthe protective powder in an amount of from 3 to 20% by weight, whereinsaid sintering additives are selected from the group consisting of MgO,Y₂ O₃, Mg₃ N₂, CeO₂ and mixtures thereof with Fe.
 23. The process ofclaim 16, wherein the reaction bonded silicon nitride article product isstructurally and chemically homogeneous.
 24. The process of claim 16,wherein the article is embedded in the protective powder by the stepswhich consist of placing the article on a layer of the protective powderand then embedding the article in the protective powder.
 25. The processof claim 16, wherein the protective powder consists of silicon nitrideand the one or more sintering additives.
 26. The process of claim 16,wherein the protective powder consists of boron nitride and the one ormore sintering additives.
 27. The process of claim 16, wherein theprotective powder consists of silicon nitride, boron nitride and the oneor more sintering additives.
 28. The process of claim 22, wherein thereaction bonded silicon nitride article product is structurally andchemically homogeneous.
 29. The process of claim 22, wherein the articleis embedded in the protective powder by the steps which consist ofplacing the article on a layer of the protective powder and thenembedding the article in the protective powder.
 30. The process of claim22, wherein the protective powder consists of silicon nitride and theone or more sintering additives.
 31. The process of claim 22, whereinthe protective powder consists of boron nitride and the one or moresintering additives.
 32. The process of claim 22, wherein the protectivepowder consists of silicon nitirde, boron nitride and the one or moresintering additives.